Apoptosis is a highly regulated program of cell death. Recent studies suggest that apoptosis is an important mechanism of tissue injury in allograft rejection. The hypothesis to be tested is that apoptosis has a dual role in the allograft, a deleterious role is a mechanism of graft cell damage and a beneficial role involving elimination of alloreactive cells. The overall goal of this project is to modulate apoptosis during organ transplantation so as to inhibit death of graft associated cells and to promote deletion of infiltrating host reactive cells. An integrated series of studies utilizing well defined animal models of orthotopic liver transplantation and small intestinal transplantation are proposed. In Specific Aim 1 the mechanisms responsible for apoptotic graft damage during allograft rejection will be defined. RNase protection assays, and competitive reverse transcriptase polymerase chain reaction will be utilized to determine the cellular and soluble initiators as well as the signaling pathways which culminate in irreversible cell damage and death. Further studies will utilize functional assays to examine the proteolytic cleavage of caspases during graft rejection. Specific peptide inhibitors of caspase activation will be administered to allograft recipients to disrupt the apoptotic pathway and graft damage. The investigators have previously determined that CD8+ cells are not essential for allograft rejection or graft cell apoptosis. The apoptotic pathways that function independent of CD8 cells will therefore be elucidated and modulated in Specific Aim 2. In Specific Aim 3 they will examine the role of apoptosis in inducing allograft tolerance. Recently, they have determined that depletion of the majority of host CD4+ T cells results in abundant apoptosis of graft infiltrating cells with minimal allograft damage and rejection. The cellular and biochemical mechanisms by which infiltrating lymphocytes are deleted will be examined using this model. Furthermore, the impact of conventional immunosuppressives and the deletion of alloreactive host lymphocytes will be studied. Definition and characterization of apoptotic pathways in organ transplantation will lead to novel therapeutics to enhance graft survival and eliminate alloreactive cells while preserving the host immune response.